This invention pertains generally to the field of communication systems and particularly to multi-carrier code-division multiple access and time division multiple access wireless communication systems.
There is an ever increasing demand for higher data rates in wireless communication systems. Future wireless personal communications services will require higher bandwidths than presently available to allow multimedia services to be offered by wireless service providers. A conventional approach to allowing access by multiple users to wireless communication systems is code-division multiple access (CDMA). CDMA systems allow several users to simultaneously and asynchronously access a wireless communications channel by modulating and spreading the information bearing signals from each user utilizing pre-assigned code sequences. Conventional CDMA systems are fundamentally limited in their ability to deliver high data rates due to implementational issues associated with higher chip rates, as well as complexity issues relating to higher Inter-Symbol Interference (ISI). These limitations severely constrain the data rate supportable by single carrier CDMA systems.
Multi-carrier modulation schemes, often referred to as Orthogonal Frequency-Division Multiplexing (OFDM), have the ability to support high data rates while ameliorating ISI and fading. The OFDM modulation scheme splits the high rate data stream into a number of parallel, lower rate data streams that are transmitted over narrow band orthogonal subcarriers. The longer symbol duration of the lower rate data stream significantly reduces the effects of ISI. Using such schemes, ISI can be eliminated by adding a guard time (often called a cyclic prefix) between the different symbols that is larger than the multi-path spread experienced by each narrow band channel. An attractive aspect of OFDM is that its modulation and demodulation can be implemented efficiently by the Discrete Fourier Transform (DFT).
A multi-carrier CDMA scheme (MC-CDMA) has been proposed which is based on the combination of CDMA and OFDM to support higher data rates in CDMA systems. See, K. Fazel, et al., xe2x80x9cOn the performance of convolutionally-coded CDMA/OFDM for mobile communication systems,xe2x80x9d Proc. IEEE PIMRC, September 1993, pp. 468-472; J. P. L. N. Yee, et al., xe2x80x9cMulticarrier CDMA in indoor wireless networks,xe2x80x9d Proc. IEEE PIMRC, September 1993, pp. 109-113; and E. Sourour, et al., xe2x80x9cPerformance of Orthogonal Multi-Carrier CDMA in a Multi-Path Fading Channel,xe2x80x9d IEEE Trans. Commun. Vol. 44, March 1996, pp. 356-367. The lower data rate supported by each subcarrier is manifested in longer symbol and chip durations. Hence, the MC-CDMA system encounters reduced ISI and frequency selectivity. In a properly designed MC-CDMA system, each single subchannel encounters flat fading, thereby eliminating the need for channel equalization. Furthermore, such systems exploit frequency selective fading for diversity through the different subcarriers. MC-CDMA also requires lower speed parallel-type digital signal processing to deliver system performance comparable to single-carrier CDMA systems, which require a fast serial-type signal processing. These important features make MC-CDMA a strong candidate for future high rate wireless communication systems.
Although the OFDM scheme is robust with respect to ISI, its performance and ease of implementation critically depends on orthogonality between subcarriers. The non-ideal system characteristics encountered in practice destroy the orthogonality between the different subcarriers. These non-ideal conditions include frequency offsets and phase noise, due to the inefficiency of the transmitter and/or the receiver, as well as Doppler effects due to fast fading. MC-CDMA systems are more sensitive to these imperfections than single-carrier CDMA (SC-CDMA) systems because of the longer symbol durations in MC-CDMA systems. For example, the communications channel may appear almost constant over one (relatively short) symbol duration in a SC-CDMA system, but may exhibit faster fading over one (relatively long) symbol duration in a MC-CDMA system. Loss of orthogonality between the different subcarriers is due to the dispersion of signal power in a particular subcarrier into adjacent frequencies. This also results in leakage between subcarriers, causing Inter-Carrier Interference (ICI) that further degrades performance. Since existing MC-CDMA receivers process each subchannel separately, they collect only part of the transmitted power in each carrier, in addition to suffering from ICI. The degradation in performance due to these imperfections has severely limited the practical use of MC-CDMA. See, e.g., P. Robertson, et al., xe2x80x9cAnalysis of the Loss of Orthogonality Through Doppler Spread in OFDM Systems,xe2x80x9d Proc. GLOBECOM 99, IEEE, Brazil, December 1999, pp. 1-10; L. Tomba, et al., xe2x80x9cSensitivity of the MC-CDMA Access Scheme to Carrier Phase Noise and Frequency Offset,xe2x80x9d IEEE Trans. Veh. Technol., Vol. 48, September 1999, pp. 1657-1665.
In accordance with the invention, reception of multi-carrier signals, such as in MC-CDMA, is carried out in a manner which not only is less sensitive to imperfections in the communications channel and local oscillators, but exploits the effect of some of these imperfections to improve the accuracy of the received and decoded data even relative to an ideal system without any imperfections. The receiver exploits fast fading to achieve a higher level of diversity to combat fading, and fully compensates for Doppler and frequency offsets as well as phase noise, thereby eliminating the performance loss due to these factors.
The multi-carrier receiver and the method in accordance with the invention accounts for the dispersion of signal energy from a subcarrier to one or more adjacent subcarriers that results from imperfections such as fast fading, Doppler and frequency offsets, and phase noise. If the communications channel were perfect, and none of these effects occurred, each subcarrier frequency received by the receiver would contain only the information that was transmitted at that subcarrier frequency by the transmitter. However, under practical conditions, such as in mobile wireless communications systems and due to imperfections in local oscillators, the communications channels are not perfect, and data information encoded on one subcarrier frequency will disperse to other subcarriers. As a result, the conventional detection of each subcarrier separately may be contaminated and degraded, potentially yielding erroneous decoded data. In the present invention, the information originally encoded on a particular subcarrier that has spread to other subcarriers is recovered in the receiver by jointly processing both the specific subcarrier and at least one adjacent subcarrier to provide a combined signal which recovers all the signal energy that is not captured by the specific subcarrier by itself, and then decoding the combined signal to provide an improved estimate of the bit value that was encoded on the subcarrier at the transmitter.
In a multi-carrier receiver of the invention that may be utilized for receiving MC-CDMA signals, decoding is carried out for each subcarrier frequency in the received signal by projecting the received signal onto the subcarrier and onto one or more selected adjacent subcarriers. The signals resulting from the projection are combined and decoded to provide a detection statistic signal. The detection statistic signal is evaluated to determine an estimated bit value over each bit length in the transmitted signal. The estimated bit value is evaluated as the sign of the real part of the detection statistic over each bit length. The decoder for each user applies a decoding sequence for that user across different detection statistics that decodes selected encoded information in the transmitted signal on the communications channel. For example, where several users are utilizing the communications channel, as in wireless communications, each user will be assigned a distinct code so that the encoded signals of the various users are orthogonal. The transmitter encodes the several subcarriers with the particular user""s code, and the receiver for that user utilizes that code to decode the detected signal information.
The invention may also be incorporated in a modified MC-CDMA communication system in which the original data at the transmitter is converted from serial data having a bit length T to M parallel bits, each having a bit length MT. These multiple bits are then encoded in accordance with the user""s code onto multiple subcarriers. The received signal is detected by the receiver for each subcarrier in the transmitted signal to provide M estimated bits in parallel, which may then be converted from parallel to serial data at the output of the receiver.
In an MC-CDMA communication system, the transmitter may utilize subcarriers separated from each other in frequency by 1/T, where T is the bit duration, or the subcarriers may be separated in frequency by 2/T. In the former case, the receiver utilizes a decoder for each subcarrier frequency in the transmitted signal. In the latter case, the receiver may utilize joint processing only of adjacent subcarrier frequencies for subcarriers that are actually transmitted (the xe2x80x9cactivexe2x80x9d subcarriers), or may process the adjacent active subcarriers and also the so-called xe2x80x9cinactivexe2x80x9d subcarriers, that are at frequencies between the frequencies of the active subcarriers, to improve the accuracy of the data provided from the receiver at the cost of slightly higher complexity.
The number of adjacent subcarriers around a particular subcarrier on which the received signal is projected is preferably selected in accordance with the type of imperfection in the communications channel that is encountered. Generally, most of the signal information will be concentrated in a few adjacent subcarriers, typically five or less above and below the frequency of the subcarrier frequency for that specific decoder. Generally, in the presence of imperfections, the received signal is preferably projected onto at least one subcarrier above or one subcarrier below the subcarrier frequency for the specific decoder.